main unit has an expansion bus bay area, pinout is here;
This connects to an IFU-30 bay, which contains an μPD91061GD;
Finally, the C-CD connector goes to the actual PC8801-30 CD drive, part list.
Practically, both pc8801mc and pce cdsys have limited host SCSI interface:
BSY, REQ, MSG, I/O, CD as input;
SEL as output;
On boot they put 0x81 -> 0xff on the data bus then pings the SEL line, this implies that the SCSI bus is configured to select ID devices 0 and 7. Speculation is that '91061 is a SCSI bridge mapped at 7 that is responsible for details towards device 0.
angelosa
commented
1 year ago 
PC Engine PCB flow is:
Practically, both
pc8801mcandpce cdsyshave limited host SCSI interface:On boot they put
0x81 -> 0xffon the data bus then pings the SEL line, this implies that the SCSI bus is configured to select ID devices 0 and 7. Speculation is that '91061 is a SCSI bridge mapped at 7 that is responsible for details towards device 0.